Electric amplifier system



March 30, 1943. H. BOUCKE ELECTRIC AMPLIFIER SYSTEMS,

Filed march 29, i940 2 Sheets-Sheet 1' as, P

2" 57 +z5ai r IINVENTOR BY Heinz Boy-ale ATTORNEY March 30, 1943. H.BoucKE I ELECTRIC AMPLIFIER SYSTEMS- 2 heetssneat 2 Filed March 29, 1940v INVENTOR Y Jinz Bouckc,

ATTORNEY Patented Mar. 30, 1943 ELECTRIC r 3 EB SYSTEM ration of NewYork Application March 29, 1940, Serial No. 326,639 In Germany ch 27,1939 12 Claims.

This invention relates to electric amplifiers serving for strengtheningweak signal variations of both low and high frequency for use in radiosignaling, talking picture, public address and various other systemsknown in the art.

More particularly the invention is concerned with amplifiers of the typecomprising means for controlling the amplification or a differentcharacteristic of the output signals in dependence upon a condition oranother characteristicof the signals beingamplified;

Among numerous uses, the invention has special application in feedbackamplifiers comprising an arrangement for varying the degree of feedbackby controlling the amplification of a valve serving simultaneously foramplifying the useful (signal) variations.

In arrangements of the above type the change in feedback due tovariation of the mutual conductance or amplification of the valve isaccompanied by a substantial variation of the amplification of theuseful signal variations. In radio frequency amplifiers havingcontrolled feedback for band width or fidelity control the abovedrawback may be eliminated by employing a con-' trol potential varyingaccording to the average amplitude of the radio signals (AVC potential)for controlling the feedback. In low (audio) frequency feedbackamplifiers an arrangement of this type for controlling the feedbackcannot be employed in most practical cases.

Accordingly, an object of the present invention is to provide a methodof and means for varying the degree of feedbackin an amplifying stage byvarying the amplification of the feedback potential or currentsubstantially without interfering with or affecting the amplification ofthe useful (signal) oscillations.

With this general object in view, the invention involves the use of anamplifying valve in' a feedback amplifying stage comprising in additionto the cathode and the usual control grid at least two furtherpositively biased or current conveying electrodes (anodes) the currentsto which are controlled by an additional control or regulation grid orany other controlling elee ment in a different sense or at least to adifferent degree. The circuit connected to one of the positiveelectrodes advantageously includes the coupling elements for translatingthe signal variations to a succeeding amplifying stage or translatingdevice, while the circuit connected to the other positive electrodepreferably serves to supply the feedback potential to be impressed uponthe input grid circuit or upon another preceding circuit of theamplifier system.

Accordingly, a more specific object of the invention is to provide asingle feedback amplifier stage for both positive and negative feedbackand suited for both. low (audio) frequency and high frequencyamplification, said stage embodying means for controlling the amount ordegree of feedback substantially without affecting the amplification ofthe useful signal variations.

Another object is the provision of an improved feedback amplifierembodying resonant coupling means in the input and output circuitsthereof, wherein the phase of the feedback potential or current issubstantially unaffected by variations of the signal frequency and thecharacteristics of said coupling means.

A further object is to provide means for and a method of effective bandwidth or selectivity control in an amplifier for low frequency ormodulated high frequency signal energy substantially without affectingthe operating characteristics, in particular the amplification of theamplifier.

Another object is to effect automatic selectivity (background noise)control in an audio frequency amplifier without affecting the operatingcharacteristics, in particular the amplification of the useful signals.

A further object is to provide a novel construction for an amplifiervalve especially suited for the purposes of and adapted for embodimentin the circuits according to the invention.

The above and further objects of the invention will become moreapparent'from the following detailed description taken with reference tothe accompanying drawings forming part of this specification andwherein;

Figure 1 is a circuit diagram for a high frequency amplifier embodied inaudio receiver with controlled feedback for effecting a variable bandwidth or fidelity control utilizing the prin ciple of the invention,

Figure 2 is a diagram for an audio frequency amplifier especially suitedfor use in connection with sound pick-up devices and embodying acontrolled feedback system for effecting automatic background noisecontrol in accordance with the invention,

Figure 3 shows a modification including features of improvement of anamplifier of the type according to Figure 2,

Figure 4 represents a schematic diagram of an amplifying valve structuresuited for use in the crcuits shown in the previous figures,

' to Figure 2 but embodying a valve of the type shown in Figures 6 and7.

Figure. 5. is an isometric view with parts broken In an arrangement ofthe aforedescribed type, if a regulating (bias) potential Er applied tothe grid 20 is varied towards negative values such 7 as by the aid Of apotential divider 32 shunted Like reference numerals identify like partsthroughout the different views of the drawings.

Referring to Figure 1, there is shown a diagram for a superheterodyneradio receiver embody ns an Iistage with controlled feedback forfidelity or band width control in accordance with the invention. Theremaining portions of the receiver are shown schematically only beingunnecessary for an understanding of the inven-' tion. An input circuit(not shown) connected between antenna l and ground ll serves to re-'ceive input signals applied to an RF amplifier and first detector ormixer collectively shown by the rectangle l2 and well known to thoseskilled in the art. Intermediate frequency signals supplied by the firstdetector are impressed upon the grid-cathode path of the multiple grid,in-

termediate frequency amplifying valve l byway of a band-pass filtercomprising the primary and secondary resonant circuits l3 and H. Thevalve 45 forming the control stage according to the invention furthercomprises a cathode l6 followed in succession by a signal control gridH, a screen grid It at zero alternating and positive direct potentialrelative to the cathode, a first grid shaped anode referred to as anodegrid It, a further control electrode referred to as current distributioncontrol or regulation grid 20, and a second anode or plate 2|. AmplifiedIF signals are derived from the first anode or grid is by way of aband-pass filter comprising resonant primary and secondary circuits 22and 23, respectively, and impressed upon a succeeding IF stage ordirectly upon the second detector and audio frequency Lamplifier ofstandard design collectively shown by the rectangle 29; The amplifiedaudio signals are applied to a suitable. translating device such as aloud speaker 30. Item 28 represents a netacross a biasing potentialsource 3|. the amount of feedback current conveyed to the anode 2| willbe decreased. At the same time, the current conveyed to the circuit ofthe and I 9 and consequently the amplification in this circuit will beincreased, thereby effecting a compensation for the increased damping ofthe input circuit l4 due to the decreased feedback or regeneration.

By a proper choice of the working point upon the operatingcharacteristic for the grid 20 and design of the circuit constants, theconditions for a practically constant amplification may be established.

A circuit of the type described may serve for remote control of the bandwidth or selectivity by placing th potential divider at the distantcontrol point. shunting condenser 33 serves to eliminate disturbancesand interfering potentials.

- The effect of band width control may be improved by the employment ofa negative or inverse feedback circuit in addition to and controlled inan opposite sense to the positive feedback. In the example shown thereis provided I for this purpose a further feedback path con workcomprising a resistance shunted by a condenser in the'cathode returnlead to provide proper operating bias for the grid I! in accordance withstandard practice. Suitable positive potential is applied to the gridsI8, I!) and to the plate2l in a well known manner such as by way of atapped potential divider or the like indicated by the plus signs in aconventional manner.

Feedback signal potential is generated at the anode 2| by the action ofa suitable coupling impedance such as ohmic resistance 24 connectedbetween the anode and the source of positive potential supply; Thefeedback potential is impressed upon the grid input circuit H by way ofa feedback circuit shunted across the anodecathode path of the valve andcomprising a condenser 28 and feedback coil 25 arranged in inductivecoupling relation with the inductance of the input circuit H. The ohmicresistance 24 may be replaced'by an inductive impedance as is readilyunderstood. Condenser 28 may be variable. for adiusting the feedback orfrequency discriminating characteristic of the feedback circuit.

nected between the positive grid l9 and the cathode and comprisingcondenser 36 in series with feedback coil 35 arranged in inductivecoupling relation with the input circuit It in such a manner as tocounteract the direct' feedback produced by coil 25. The simultaneoususe of both positive and negative feedback controlled in an oppositesense by the regulating potential on the grid 20 on account of theopposite effect of the latter on the electrodes l9 and 2| makes itpossible, provided a suitable (symmetrical) design of the feedbackcircuits, to render the degree of damping reduction of the input circuitsubstantially independent of ageing phenomena of the cathode (variationof the emission current) as well as of variations of the fluctuations ofthe operating potential especially of the anode supply voltage and otherinfluences, inasmuch as the latter will affect both output circuits in alike manner and produce feedback effects cancelling each other incontrast to the feedback current variations due to the control by thegrid 20. This is of special importance if a maximum and stabilizeddamping reduction is desired.

The circuit according to Figure l, in addition to the independence ofthe amplification from the degree of feedback, has the further advantagethat the feedback potential is not derived from. an oscillatory orresonant circuit, thereby eliminating the danger of self-oscillation andtendency to instability inherent in the known feedback arrangements inhigh frequency systems.

If desirable, a further screen grid may be arranged between theregulation grid 20 and either of the adjacent positive electrodes(anodes) I9 and 2| in order to minimize or eliminate the influence ofthe feedback control potential upon either of the output circuits,especially the circuit coupled to the succeeding amplifying stage. Anexemplification of an arrangement of this type is shown in Figure 2. e

The latter represents a two-stage audio frequency amplifier embodying afrequency band width or pass range control in the first stageconstructed in accordance with the invention. The

input audio signals which may be supplied fromsistance 42which may bereplaced by an inductance coil-connected between the grid I8 and thehigh potential supply source designated as +200 in 'the drawings. Thesignal potential variations set up at the grid it are applied 'by way ofcoupling condenser 43 and grid leak resistance at to the grid-cathodepath of the succeeding amplifier valve $5 for further amplificationbefore being impressed upon the loud speaker 53 or any other translatingdevice. Valve 65 in the example shown is of the tetrode type comprisinga cathode it, input control grid 41, a screen grid 48 and plate or anoded5. Item 50 represents-a biasing network in the cathode lead similar tonetwork 26 for the valve I5. The amplified audio signals are applie tothe loud speaker 53 by way of secondary winding 52' of an audiotransformer 52 having its primary winding inserted in the anode circuitof tube 45. The grid IQ of valve I5 is maintained at a positive directpotential (+100 in the example shown) and at zero alternating potentialwith respect to the cathode to prevent reaction between grid and thecircuit connected to the grid it in such a manner that the grid 20 willnot affect the amplification of the main or useful signals in the outputcircuit of grid I8. Negative feedback potential set up at the anode 2|by means of load impedance 56 preferably an ohmic resistance as shown isimpressed upon the input grid ll by way of condenser 55 in series withresistance 54 connected between the anode 2! and the grid ll wherebyresistances 54 and 6! form a potential divider for introducing thenegative feedback potential into the input circuit. Condenser 55 is sodimensioned that the negative feedback is predominantly effective forthe higher audible frequencies.

In an arrangement of the aforementioned type, a decrease of the negativepotential on the control grid 20 will result in an increased suppressionof the higher audible frequencies. This effect is utilized to obtain avariable bandwidth or pass range adjustment such as for control of thebackground noise or needle scratch suppression in an amplifier connectedto a sound reproducing device in dependence up n the momentary strengthof the sound signals. For this purpose a portion of the output energyderived in the example shown by way of tertiary winding 52" of outputtransformer 52 energizes a rectifying circuit comprising a rectifier 51in series with load resistance 59 shunted by a smoothing condenser 58.The rectified potential developed by the re- 'sistance 59 serves tocontrol the bias of regulation grid 20, the rectifier 51 being connectedin such a manner that the bias potential is negative with respect to thecathode as indicated in the drawings.

With the rectifier 51 connected in the manner shown, the operation ofthe system will be as follows; assuming the bias on the grid 20 whichmay have an additional fixed negative bias (not shown) applied to it, toresult for medium average sound current intensity in a determined amountof inverse feedback current through the circuit ll, 54, 55 and acorresponding degree of noise reduction and linearity of theamplification characteristic. If the sound intensity increases beyondthis value, the bias potential at the grid 20 will become more negativedue to the increased potential drop across resistance 59, resulting in aproportionate blocking of the negative feedback current conveyed to theplate 2|. As a result thereof, the efiective frequency pass range of theexmplifierwill be widened due to the decreased negative feedback for thehigher sound frequencies, resulting in greater fidelity and increasednoise, the latter being no longer objectionable inasmuch as it will bedrowned out or blanketed by the increased' sound intensity. Conversely,if the sound intensity decreases below the above value, the negativebias on the grid 20 will become less, resulting in an opening up of theanode (negative feedback) path and a proportionately increasedsuppression of the higher ioufd (noise) frequencies by the negativefeed- 2 ac In known arrangements for inverse feedback, especially inaudio frequency. negative feedback amplifiers, great difficulties areexperienced due to the inductive and/or capacitative load-in the outputcircuit, whereby the feedback potential to be superimposed upon theinput potential may have its phase rotated to a considerable extent.Since the phase rotation is dependent on frequency, the effects of thenegative feedback such as distortion equalization, noise reduction,etc., largely depend upon the frequency or frequency characteristics ofthe input signal and the load or coupling circuits of the amplifier withthe effeet that the inverse feedback may be changed into a positive ordirect feedback for certain frequencies or groups of frequencies,resulting in al stability of the amplifier. This drawback is greatlyminimized or eliminated by airangements proposed by the inventionwherein the negative feedback potential is derived from a separateauxiliary anode arranged in such a manner that it is subject totheinfluence of the main control grid II in the same manner as the mainor output anode, but is substantially unaffected by the signal potentialvariations on the main anode. This auxiliary anode in general may be ofsmall dimension compared with the main anode since it only serves toproduce the feedback control without involving substantial energyexpenditure,

whereby it will collect only a small percentage of the total spacecurrent. Again in turn, only ohmic resistance may be provided in thecircuit connected to this auxiliary anode to serve as a load impedance(resistance 24 or 56), avoiding thereby any objectionable phase rotationof the feedback potential. The anode potential and anode resistance forthe auxiliary anode should be chosen in such a manner that the kind anddegree of distortion of the alternating potential set up at theauxiliary anode corresponds substantially to the distortioncharacteristic of the main anode circuit. It has been found advantageousto shield the auxiliary anode from the main anode by means of anegatively or positively :biased screen or a grid connected to acathode. This screen may be structurally combined with an existingscreen grid to form a single unit.

The control of the inverse or direct feedback may serve to effectindirectly a volume control, or to obtain simultaneous volume andfidelity control. For this purpose the feedback path in Figure 2shouldbedesigned in such a manner that the inverse feedback is eflectivefor all frequencies especially the medium range of the audiblefrequencies. If in such a system the volume is decreased by increasingthe amount oi resistance in the connection from the high potentialsource to the screen grid.

A ain in tum it is possible to eifect a control of the amplification ofthe signals being translated in the same stage such as a volume control(AVC) in dependence upon the RF signal ampli- 3 wherein a supplementaryfidelity control system is used as described in my copending applicationSerial No. 314,411 filed January 18, 1940, which matured into Patent No.2,257,782 dated October '1, 1941. Input audio signal potentials to'beamplified and derived from a suitable source are impressed by way ofinput terminals a-b 'upon the grid-cathode path of valve throughcoupling condenser 40 and grid leak resistance 4| in substantially thesame manner as shown in Figure 2. The grid IS in this exampleconstitutes a screen grid and is maintained at a fixed positivepotential.. If desired, this grid may be dispensed with. The output oranode grid l9 tude in the case of Figure 1 or for volume exension andcompression in an audio frequency amplifier of the type according toFigure 2. For this purpose the operating characteristic for the inputgrid I! should be of the exponential (variable mu) type. The potentialfor controlling the amplification such as an AVC potential is impressedupon the signal grid II in a manner well known In order to prevent thisamplification control from reacting upon the feedback circuit it isnecessary to apply an additional volume control potential to theregulation grid in such a manner as to produce an effect upon thefeedback potential opposite to that caused by the controlpotentialapplied to the input grid H. In this manner the degree offeedback remains constant and is subject only to the influence. of thefeedback control potential applied to the regulation grid 20. Moreover,the simultaneous from the high potential at the plate 2| and grid,

amplification control by the grid 20 for the output circuit 22, Figurel, entails an improvement of the AVG or fading compensation provided noscreen grid is arranged between the grids l9 and 20. In order to obtainautomatic volume control, therefore, in Figure 1 the AVG voltageproduced in the usual manner by rectification of a portion of the RFenergy is applied with its negative pole to the control grid II. Inaddition, an additional AVC voltage is applied with its positive pole tothe grid 20. The additional AVC;

voltage is preferably produced in a separate rectifying circuit andinserted at a suitable point P in the grid circuit.

According to a modified arrangement the succeeding stage may beconnected to the anode 2| and the feedback potential derived from thegrid i9. In-this case the additional AVC potential applied to the grid20 should have the same polarity as the AVG potential on the grid I! toeliminate the influence of the volume control upon the feedback. Theabove modifications are not limited to arrangements with additionalvolume control but apply in a like manner to other methods forcontrolling the amplification of the s;gnai being translated such as forvolume expension and compression in audio amplifiers of the typeaccording to Figure 2. a

According to a further feature, the arrangements described may becombined with other control circuits such as for band width or fidelitycontrol or many other uses. A combined arrangement for band width orfidelity control in. an audio frequency amplifier is shown in Figure 75is connected to the high voltage source through a pair of resistancesand GI inseries. Resistance 8| serves as a load to produce amplifiedcontrol potential at the junction between the resistances 60 and ii tobe impressed upon the grid 41 of the succeeding amplifier stage 45 byway of decoupling impedance such as an ohmic resistance 66, blockingcondenser Q8 and grid leak resistance 69. Additional potential isdeveloped.

at the anode 2| by the action of anode load resistance 63 and appliedsimultaneously to the grid 41 of amplifier 45 by way of decouplingresistance 65 and blocking condenser 61. Condensers 61 and 68 serve toisolate the grid 41 iii of valve Ii. Resistances B5 and 86 serve toprevent mutual interaction or coupling between the output circuits ofgrid 19 and plate ll of valve l5 by way of the common grid input circuitof valve 45. Resistance Si in the output circuit of grid I9 is shuntedby a condenser 62 designed so as to by-pass the high audiblefrequencies. Inverse feedback potential is applied from the grid iii tothe input grid I! by way of a feedback circuit comprising condenser 55and resistance 54. Condenser 56 is designed to predominantly pass thehigh audible frequencies whereby these frequencies are suppressed onaccount ofthe inverse feedback action. The circuit for the anode M has asubstantial linear frequency characteristic or may be designed foraccentuation of the higher notes or audio frequencies. A furtheramplification stage or a translating device connected to outputterminals c-d may be energized by the output of valve 45 as isunderstood.

In a system as described hereinbefore, if the control potential Erapplied to the regulation grid 20 by way of terminals 6-) has a highnegative value the anode path will be blocked in such on the other handthe anode path will be opened resulting in a widening of the frequencypass" range assisted by the decrease of the current to the anode i9 andconsequent reduction of the higher frequencies due to the inversefeedback.

It is to be pointed out that the regulation grid Y neednot of necessitybe arranged between the positive electrodes or anodes i8 and 20. Thelatter may both be arranged at one side of the controlgrid, it beingonly essential for the invention that the influence on both electrodesby the control grid is of an unequal nature. Thus,

for instance, Figure 2 may be modified in the following manner: Theconnection of the anode 2| is interchanged with the connection of grid19. In a modified arrangement of this type. the

inverse feedback potential derived from the grid bias which is more orless decreased by the controlling potential. In the aforementionedmodifled circuit, in order to protect the grid Hi from the influence bythe control grid, a screen gridmay be arranged between the grids l8 andiii. In arrangements of this type the anode is not absolutely necessaryif sufllciently high poten-,

tials are applied to the grids l8 and N. If the anode is omitted or leftdisconnected, the grid 20 no longer will have-the function of a currentdistribution grid but will effect regulation in such a manner that thecurrent to the grid l9 decreases the higher the negative bias on thegrid 20. In order to obtain the same effect, the polarity of the controlpotential may be the same as shown in Figure 1.

According to a further modification, the positive electrode which shouldbe substantially unaflected by the auxiliary control electrode may bedisposed betv.een the cathode and the input control grid.

In the foregoing there are described arrangements suitable for effectinga volume control being either dependent upon or independent offrequencyby varying the degree of negative feedback in an amplifier. This controlis produced by the aid of an auxiliary (regulation) grid arranged toinfluence the current to a pair of positive (anode) electrodes in adifferent sense or at least in a varying degree. The inverse'feedbackpotential is supplied by the positive electrode which is subject tointense control by the regulation grid, while the potential serving forcontrolling the succeeding stage or for energizing a translating deviceis supplied by the positive electrode not or less subject to the controlinfluence of the regulation grid, to obtain e. g. a band width orfidelity control without attendant volume changes or to effect volumecontrol exclusively by varying the degree of negative feedback.

According to a further feature of the invention, the arrangementsdescribed may serve for improved volume control in such a manner thatthe indirect volume control by varying the degree of negative feedbackis supplemented by the direct amplification control effectedsimultaneously by the same regulation electrode in the output circuitfor the useful signal variations. For this purposeit is necessary thatthe regulation electrode is disposed between the positive electrodes andthat these electrodes are'arranged in such a manner that contrary to theprevious arrangements both the auxiliary (feedback) electrode and themain output electrode are subject to eflicient control by the regulationgrid.

An exemplification of a system of this type is obtained by interchangingthe connections of the grids l8 and IS in Figure 2 and by designing thecondenser 55 in the negative feedback circuit in such a manner as topass all audible frequency freely. In this case the control by the grid20 will be such that an increase of the inverse feedback potential isaccompanied by a decrease of the amplification in the circuit of thegrid l9;

subject to the phase reversal.

that is, the output circuit for the signal varia-' tions. Both effectsof the regulation potential will produce a multiple total effect on theoverall amplification by the stage. Thus, if the variation due to thenegative feedback has a range of 1-20 and the direct regulation in'thesignal output a range of 1-5 or 1-10, then the actual total regulati 1grange will be from 1-100 to 1-200, respectively.

In order to effect volume control in a high frequency amplifier in whichcase an efllcient and intensive regulation is desirable, the circuitaccording to Figure 1 may be modified by changing the polarity of thecoil 25 so as to produce an inverse or negative feedback and to connectthe coil 35 so as to produce a direct feedback or in other words byinterchanging the positive and negative feedback circuits. In such acircuit, due to the opposite effect of the separate feedbacks incombination with the direct control of the amplification in the outputcircuit of electrode I9, a very high sensitivity and increased range forthe regulation are obtained.

An advantage of the proposed new regulating arrangement resides in thefact that improved regulating action is obtained without requiring acurved grid (variable mu) operating characteristic as in the case ofthetknown regulating valves. It is therefore advisable to employ a tubewith linear characteristic for volume or gain control in circuitarrangements of the type according to the invention.

In arrangements wherein the two output electrodes (anodes) are disposeddirectly at opposite sides of the auxiliary regulation grid, it has beendiscovered that space charge effects may occur, whereby with increasingnegative control potential the inverse feedback'potential at the anodewill at first become zero, then reverse its phase and increase again, orin other words, that the h inverse feedback becomes weaker and isfinally converted into positive feedback. In order to obviate thisdrawback, thezfollowing threeprocedures may be followed: (1) Thenegative feedback potential is derived from the grid I9 located at theside of the cathode which is not v In this case the control potentialshould not become excessive so as to cause phase reversal of the anodepotential which in this case serves to supply the output for thesucceeding stage; (2) limitation of the space charge near the grid I9 bysuitable construction of the electrodes and choice of the operatingpotentials; (3) selection of the operating conditions' in circuitsaccording to Figures 1 and 2 so that the regulation takes place withinthe space charge region, whereby beginning with a certain regulatingvalue the inverse feedback potential decreases and is changed intopositive feedback. The sense of regulation does not change in this caseand a more intensive volume control is obtained especially inhighfrequency stages,

eIn carrying out the invention as described hereinbefore, double controlamplifying valves of I little or not at all aflected by the controllingaction of there gulation grid or other controlling element.

Accordingly, the special tube constructions proposed by the inventionand suited primarily for the purpose of the invention are character-.ized by the provision of at least two anodes having like or differentload or current conveying capacity, the current to which is controlledby at least one controlling grid in an opposite sense or to a varyingdegree; that is, in thecase of anodes of different load capacity in sucha manner that the anode of higher load capacity is not at all or only toa slight extent subjected to the controlling action when compared withthe anode of lower load capacity.

According to one embodiment, the above object is accomplished byarranging one of the anode electrodes in front of the regulation grid atthe side of the cathode, while the other anode of like or preferablylower load capacity is dis posed at the other side of the regulationgrid. A schematic arrangement for an electrode structure of this type isshown in Figure 4, wherein like reference numerals are employed as inthe preceding figure. The anode of high load capacity has an outer solidportion l9 and a central foraminous portion l9, while the anode of lowerload capacity 2| (feedback anode, etc.) ,is substantially equal to inarea and disposed opposite the foraminous portion IQ of the electrodel9. In this manner, the effect of the controlling grid 20 on theelectrode I9 is greatly minimized or suppressed for practical purposes.

A practical electrode structure of this type comprising cylindricalelectrode elements is shown in Figure 5, the details of which areselfevident from the foregoing,

According to a modification, theoutput electrodes or anodes may bearranged injuxtaposition as shown schematically in Figure 6. In thelatter two anodes 2| and 2|, the former being designed for high loadcapacity and the latter for low load capacity, are arranged side by sideand screened from each other to prevent interaction by means of a screenelement 10. The 'latter may be connected to the cathode It to serve as asuppressor grid or it may be maintained at a high positive potential andfor this purpose structurally combined with an existing screen grid suchas the grid I 8 in the example shown. I

A structural embodiment of a tube of this type is shown in Figure '7,comprising cylindrical elegtrode elements the details of which will beselfevident from the fore oing. Instead of arran ing themain anode 2|and auxiliary anode 2| at the same distance from the cathode bothelectrodes may be staggered to secure desired operating characteristics.The control grid 20 in this case mainly affects the auxiliary anode 2|in a manner similar as in the case of the construction according toFigures 4 and 5. Additional electrodes such as screen grids may beprovided-between the several electrodes as will be obvious from theabove. 1

Referring to Figure 8, there is shown a low (audio) frequency amplifierwith controlled inverse feedbackto obtain automatic volume control. Thiscircuit corresponds substantially to the circuit of Figure 2 with'theexception of the employment of an amplifying valve of the type describedand shown by Figures 6 and 7. The 7 main output (signal) potential issupplied by the anode 2| and applied directly to the loud speaker 53 byway of audio transformer 52. The negative feedback potential is set upat the auxiliary anode 2| by the action of load resistor 56 and isapplied to the input circuit in the manner described hereinbefore. Theamount of negative feedback is controlled by the regulation grid 10substantially independentlyof the amplification of the main or usefulsignals in the output circuit of anode 2| by means of a controllingpotential. obtained by averaging a portion of the output energy in arectifying and smoothing system sistance .59 is superimposed with itspolarity reversed compared with the arrangement shown in Figure 8.

According to a further feature, it is possible to render the volumecontrol dependent on frequency by a corresponding design of the couplingpath.

Such an arrangement according to Figure 8 has i the advantage thatvolume expansion may be effected by means of a control potential in apower amplifier which is not possible with power amplifiers of thepresent known type without entafling additional distortion. Such aregulation is especially desirable if the final stage is directlyconnected to the detector of a radio receiver without a previous audio(driver) stage. A further advantage is due to the fact that the A. C.

Y hum of a stage when adjusted to low amplificaare derived from separateanodes.

tion is reduced to a considerable extent.

As is understood, the circuits for positive or negative feedback may beemployed with a constant or fixed degree of feedback or regeneration inwhich case the auxiliary regulation grid 20 and associated controlcircuits may be omitted. Arrangements of this type have the advantagedescribed above over the existing inverse feedback systems that thefeedback potential is derived from a separate output or anode electrodeby the aid of a non-reactive coupling element such as an ohmicresistance 58 thereby substantially avoiding frequency discriminationand eliminating the danger of phase rotation and consequent tendency tooscillate and instability of the am- Amplifying valves constructedaccording to the invention may also be employed in reflex stages forsimultaneous amplification for high frequency and low frequency signals.In this case, the high frequency and low frequency potentials areimpressed upon the same input grid (grid II) but Thus, if the highfrequency is derived from the anode 2| (Figure 8), volume control forthe high frequency potential for fading elimination, etc., maybeemployed without affecting the low frequency amplification.

When using valves the anodes of which are designed for equal orsubstantially equal load capacity for fidelity control the anodepredominantly subjected to the action of the regulation grid preferablyserves to supply a circuit designed speaker is energized from the otheranode. By

controlling the current conveyed to the first anode there is obtained inthis manner a desired ratio between the high and low audible frequenies.

A low frequency power stage embodying a valve of the type described mayalso be employed for improving the. volume control or fading eliminationby impressing an AVC voltage with its positive side to the control grid20 to effect a volume control by varying inverse feedback. If in such anarrangement the negative side of the AVG voltage is at the cathode it isnecessary to produce the control potential for the inverse feedback bythe aid of an additional rectifier circuit energized by a portion of thehigh frequency energy, or to reverse the sense of variation of thecontrol potential by a phase reversing circuit.

It will be evident from the above that the invention is not limited tothe specific details of construction and arrangement of parts shown anddisclosed herein for illustration but that the underlying idea andprinciple of the invention are susceptible of nume' )ilS variations andmodifications coming within the broad scope and spirit of the inventionas defined in the appended claims. The specification and drawings areaccordingly to be regarded in an illustrative rather than a limitingsense.

I claim:

1. In an amplifier, an amplifying valve comprising cathode, signalcontrol, main anode, auxiliary anode, and regulation electrodes, spacecurrent supply means for said anodes, said electrodes being relativelyarranged so that said signal control electrode is effective in,controlling the space current to both said anodes and said regulationelectrode is effective in controlling the space cur-- rent to saidauxiliary anode only, signal input and output circuits operativelyconnected to said signal control electrode and said main anode, re-

spectively, and feedback circuit means between the auxiliary anode andsaid input circuit.

2. In an amplifier, an amplifying valve comprising cathode, signalcontrol, main anode, auxiliary anode and regulation electrodes, spacecurrent supply means for said anodes, said electrodes being relativelyarranged so that said signal control electrode is effective incontrolling the space current to both said anodes and said regulationelectrode is effective in controlling the space current to saidauxiliary anode only, signal input and output circuits operativelyconnected to said signal control electrode and said main anode,respectively, inverse feedback circuit means between said auxiliaryanode and said input circuit, and means for applying variable biaspotential to said regulation electrode.

3. In an amplifier, an amplifying valve comprising cathode, signalcontrol, main anode, \auxiliary anode and regulation electrodes, spacecurrent supply means for said anodes, said electrodes being relativelyarranged so that said signal control electrode is effective incontrolling the space currents to both said anodes and said regulationgrid is effective in controlling the space current to said auxiliaryanode only, signal input and output circuits operatively connected tosaid signal control electrode and said main anode, re-

spectively, substantially non-reactive load impedance means connected tosaid auxiliary anode, inverse feedback means between said auxiliaryanode and said input circuit, and means for impressing a variable biaspotential upon said regulation electrode.

4. In an amplifier, an amplifying valve comprising cathode, signalcontrol grid, main anode, auxiliary anode and regulation electrodes,space current supply means for said anodes, said electrodes beingrelatively arranged so that said signal control electrode is effectivein controlling the space currents to both said anodes and saidregulation grid is effective in controlling the space current to saidauxiliary anode only, signal input and output circuits operativelyconnected to said signal control electrode and said main anode,respectively, anrohmic load impedance connected to said auxiliary anode,inverse feedback means between said auxiliary and said input circuit,and means for impressing a variable bias potential upon said regulationelectrode.

5. In a high frequency amplifier, an amplifying valve comprisingcathode, signal, control,

main anode, auxiliary anode and regulation electrodes, space currentsupplymeans for said anodes. said electrodes being relatively arrangedso that said signal control electrode is effective in controlling thespace current to both said' anodes and said regulation electrode iseffective in controlling the space current to said auxiliary anode only,tuned high-frequency signal input and output circuits connected to saidsignal control electrode and saidmain anode, respectively, a periodicload impedance means connected to said auxiliary anode, feedback circuitmeans between said auxiliary anode and said input circuit, and means forimpressing a variable bias potential upon said regulation electrode.

6. In an audio frequency amplifier, an amplifylng valve comprising acathode, a signal control grid, a main anode, an auxiliary anode and aregulation grid, space current supply means for said anodes, meanswhereby said signal con trol grid is effective in controlling the spacecurrent conveyed to both said anodes and whereby said regulation gridcontrols substantially the space current to said auxiliary anode only,signal input and output circuits operatively connected to the signalcontrol grid and to the main anode, respectively, an inverse feedbackcircuit designed to predominantly pass the high audible frequenciesconnected between said auxiliary anode and said input circuit, and meansfor impressing a variable bias potential upon said regulation grid.

7. In an audio frequency amplifier, an amplifying valve comprising acathode, a signal control grid, a main anode, an auxiliary anode and aregulation grid, space current supply means for said anodes, meanswhereby said signal control grid is effective in controlling the spacecurrent conveyed to both said anodes and whereby said regulation gridcontrols substantially the space current to said auxiliary anode only,signal input and output circuits operatively connected to the signalcontrol grid and the main anode, respectively, substantiallynon-reactive load impedance means connected to said auxiliary anode, aninverse feedback circuit designed to predominantly pass the high audiblefrequencies connected between said auxiliary anode and said inputcircuit, means for producing a direct potential varying in proportion tothe average,

amplitude of the signal energy being amplified, and further means forimpressing said direct po tential upon said regulation grid.

8. In an electric amplifier, an amplifying valve comprising a cathode, afirst anode, asignal control grid near saidcathode, a regulation gridnear said anode, a second foraminous anode located between said signalcontrol grid and said regulation grid, a screen grid located betweensaid regulation grid and said second anode, space current supplymeansfor said anodes, signal input and output circuits operatively associatedwith said input grid and said second anode, respectively, feedbackcircuit-means between said first anode and said input circuit, and meansfor applying a variable bias potential to said regulation grid.

9. In an audio frequency amplifier, an ampliiying valve comprising acathode, a first anode, a signal control grid near the cathode, aregulation grid near the anode and a second foraminous anode betweensaid signal control grid and said regulation grid, space current supplymeans for said anodes, signal input and output circuits operativelyconnected to said input grid and to said second anode, respectively,substanthe first anode, an inverse feedback circuit shunted across saidcathode and said first anode and including an impedance having a'tapthereon connected to said input grid, and means for impressing avariable bias potential upon said regulation grid.

11. In an'audio frequency amplifier, an amplifying valve comprising acathode, a first anode, a signal control grid near the cathode, aregulation grid near the anode, and a second foraminous anode betweensaid signal control grid tially non-reactive load impedance meansconnected to said first anode, an inverse feedback circuit designed topredominantly pass the higher audible frequencies connected between thefirst anode and said input circuit, and means for impressing a variablebias potential upon said regulation grid.

10. In an audio frequency amplifier,.an amplifying valve comprising acathode, a first anode, a signal control grid near the cathode, aregulation grid near said anode, a second foraminous anode between saidsignal control grid and said regulation grid, a screen grid locatedbetween said regulation grid and said second anode, space current supplymeans for said anodes, signal input and output circuits opera tivelyconnected to said input grid and to said second anode, respectively,substantially nonreactive load impedance means in the output of and saidregulation grid, space current supply means for said anodes, signalinput and output circuits operatively connected to said input grid andsaid second anode, respectively, substantially non-reactive loadimpedance means connect ed to said first anode, an inverse feedbackcircuit comprising a condenser in series with a re sistive impedanceshunted across the first anode and cathode, a connection from a point ofsaid impedance to said input grid, said condenser being designed topredominantly pass the high audible frequencies, means for producing adirect potential varying in proportion to the average amplitude of theaudio signals being amplified, and further means for impressing saiddirect potential upon said regulation grid to producehigh inversefeedback for low signal intensity and low inverse feedback for strongsignal intensity, respectively.

12. In an amplifier, an amplifying valve comprising cathode, signalcontrol, main anode, auxiliary anode and regulation electrode, meanswhereby said signal control electrode is effective in controlling thespace current to both said anodes and said regulation electrode isefiective in controlling the space current to said auxiliary anode only,signal input and output circuits connected to said signal controlelectrode and said main anode, respectively, substantially non-reactiveload impedance means connected to said auxiliary anode, feedback circuitmeans between said auxiliary anode and said input circuit, and means forimpressing -a variable bias potential upon said regulation electrode.

- HEINZ BOUCKE.

